Various spectroscopic techniques are routinely used to determine the constitution of chemical compositions and to monitor the progress of chemical reactions and processes. The choice of technique, including the wavelength of the radiation employed, depends on the information desired.
Infrared (IR) spectroscopy is based on the interaction with chemical substances of infrared irradiation having a wavelength between 0.77 .mu.m and 1000 .mu.m. A segment of IR spectroscopy, referred to as near infrared (NIR) spectroscopies, uses radiation wavelengths between 0.77 .mu.m and 2.5 .mu.m. IR and NIR spectroscopies generally involves the absorption of radiation as it passes through a sample. The absorption frequencies provide information regarding the chemical and physical characteristics or the molecular structure of the irradiated substance.
Ultraviolet (UV) and visible (VIS) spectroscopic methods employ UV radiation having wavelengths between 10 nm and 350 nm and visible radiation with wavelengths between 350 nm and 770 nm. UV/VIS techniques measure the absorption of the exposing radiation by molecular electronic transitions; the particular wavelengths absorbed are characteristic of the molecular structure of the substance under investigation.
Raman spectroscopy is another means by which chemical, physical, and molecular information of materials can be obtained. Incident radiation interacting with a material undergoes scattering, which occurs in all directions; the radiation may be scattered elastically or inelastically. The inelastically scattered radiation is referred to as Raman scatter. The wavelengths and intensities of this radiation comprise a Raman spectrum that provides chemical and structural information regarding the irradiated material.
Luminescence spectroscopy involves the measurement of photon emission from molecules. It includes photoluminescence such as fluorescence and phosphorescence, which are emissions from a substance resulting from its excitation by radiation absorption, and chemiluminescence, where the emission is induced by a chemical reaction. The emitted radiation is characteristic of the molecular structure.
All of these spectroscopic techniques are useful for gaining qualitative and quantitative information about a chemical material. IR, NIR, and Raman spectra, however, provide the greatest amount of molecular structural information.
Determining the constitution of a chemical composition or monitoring the progress of a chemical reaction is frequently carried out with materials situated in inhospitable environments. For example, analysis may be required of a process stream under conditions of high temperature and/or pressure or in the presence of corrosive substances or powerful solvents. It is well known to place spectrophotometric apparatus such as a spectrograph and a radiation source in a location remote from a substance that is to be analyzed in situ and connect the apparatus to the sampling site by radiation conduits comprising optical fibers. The interface between these optical fibers and the process environment is commonly provided by a probe, often referred to as a spectroscopic optical probe or a fiber optic probe.
A variety of spectroscopic probes are known in the art. U.S. Pat. No. 3,906,241, for example, describes a probe for use in analyzing fluids that incorporates three fiber optic channels, one to carry radiation from a source to the probe detecting head, a second to return radiation from the head, and a third to carry the scattered Raman radiation to detector means. In U.S. Pat. No. 4,573,761 is described a probe that comprises at least one optical fiber for transmitting light into a sample and at least two optical fibers for collecting radiation from the sample, the collecting fibers converging with the axis of the transmitting fiber at an angle less than 45 degrees. U.S. Pat. No. 4,707,134 describes a probe comprising a plurality of converging optical fibers contained in a housing that is closed at one end by a transparent window. A method for in situ detection of a compound by Raman spectroscopy is disclosed in U.S. Pat. No. 4,802,761, wherein a collecting cell is connected by an optical fiber bundle to a remote sensing device.